Fat Facts

by Len Kravitz, PhD on Sep 01, 2007

Quit debating! To burn more fat, work harder or longer—or both.

Weight management programs abound, and they employ countless strategies and approaches. But all these programs should include three principal components: (1) an exercise plan that incorporates cardiovascular and resistance training to increase caloric expenditure and maintain muscle mass; (2) a lifestyle/ dietary approach that emphasizes balanced nutrition and decreased caloric intake; and (3) a behavior modification strategy to support implementation of the exercise and lifestyle components. With fitness industry professionals, perhaps the one topic—or obsession— that will surely generate debate in the exercise component of the plan is, What is the best exercise fat-burning zone? This article will attempt to bring clarity where there is cloudiness, research where there is perception, and guidance where there is dissent on this contestable issue and related matters.

Energy Balance Basics
A kilocalorie (hereafter referred to as a calorie) is a unit of energy, and since energy is neither created nor destroyed (according to the first law of thermodynamics), the calories you eat will either be stored somewhere in your body or expended for fuel in metabolism for your daily activities, occupational tasks and exercise. This basic theory specifies that consuming more energy than you expend will lead to a positive energy balance—and weight gain. Conversely, expending more energy than you consume will lead to a negative energy balance—and weight loss. However, due to individual differences in the body's neurological, hormonal and metabolic regulatory systems, this concept of caloric balance does not work to precisely the same degree in all persons.

When a person has a negative energy balance, weight loss may come from three body sources: water, adipose tissue (fat) and muscle tissue. Under most circumstances, body water will remain relatively normal as long as regular hydration is maintained. The goal of a weight loss plan is to lose fat while preserving muscle mass.

Is Low-Intensity Exercise Better for Fat Burning?
We've all heard the claim that the best type of cardiovascular training for burning fat is lower-intensity exercise, which keeps the exerciser in the so-called "fat-burning zone." Thompson and colleagues (1998) have confirmed that at lower intensities (50% VO2max), a greater percentage of energy comes from fat than at higher intensities (70% VO2max). However, as long as workouts are the same length, the total energy expenditure will be greater, and a person will almost always burn at least as many fat calories (if not more), at a higher training intensity than at a lower training intensity. In other words, the selective use of fat as fuel that occurs in low-intensity exercise does not translate into greater fat loss. For weight loss plans, fitness professionals should focus on the exercise regime that yields the greater total volume of calories expended.

To further substantiate this association, I conducted a simple experiment. A 191-pound physically fit male student performed 30 minutes of treadmill exercise under two conditions: (1) at 55% of his heart rate maximum (HRmax) and (2) at 85% of his HRmax. The results of this experiment were as follows:

At the higher intensity, the subject burned more total calories, more fat calories and more carbohydrate calories. But not every client can exercise the way this very fit student can. For people who are sedentary or at orthopedic, cardiac or health risk, high-intensity exercise may be contraindicated. For their weight loss exercise plans, low- to moderate-intensity exercise should be performed for progressively longer durations. In fact, since most people can't do high-intensity exercise on a daily basis, owing to potential overtraining and overuse concerns, perhaps the best strategy is to integrate and balance the low- to moderate-intensity, long-duration workouts with high-intensity workouts for optimal fat-calorie burning.

Does Cardiovascular Exercise Make You a Better "Fat Burner"?
Horowitz and Klein (2000) indicate that a number of physiological and metabolic adaptations that occur with cardiovascular exercise enhance fat metabolism. These adaptations include the following:

1. An improved oxygen delivery and extraction system (via blood flow and capillarization) helps cells burn fat more efficiently.

2. The sensitivity of muscle and fat cells to epinephrine is enhanced, leading to improved release of fatty acids (which are the disassembled triglycerides) into the blood and within the muscle (where fat is in its triglyceride storage form).

3. An augmented circulatory blood flow system aids in the delivery of fatty acids to the muscle.

4. The amount of fatty acids allowed to enter the muscle increases, making more fat available for fuel.

5. An improvement in the specialized protein transporters that admit the fatty acids into the muscle cells makes the fat more readily available.

6. The mitochondria, sometimes referred to as the cells' "fat-burning furnaces," increase meaningfully in number and size.

7. The oxidative enzymes that speed up the breakdown of fatty-acid molecules to be used during aerobic exercise increase. An important take-home message for all your students and clients is that consistent, progressively challenging cardiovascular exercise will truly develop their bodies into much better fat burners.

Why is Carbohydrate the Preferred Energy Fuel?
From a caloric standpoint it seems that fat (at 9 calories per gram) should be a much better source of fuel for exercise than carbohydrate (at 4 calories per gram). However, carbohydrate is the most important fuel source for exercise and the only fuel source used proficiently in both anaerobic and aerobic training. The body prefers carbohydrate to fat during endurance exercise for two major reasons: First, the metabolic pathways for carbohydrate breakdown (glycolysis) are much more efficient than those for fat breakdown (mobilization, lipolysis and beta oxidation). Second, more oxygen is required to burn fat. The energy yield of fat from 1 liter of oxygen is 4.69 calories, as compared with a yield of 5.05 calories from carbohydrate. Thus, carbohydrate is approximately 7% more efficient as a fuel.

Where Does Resistance Training Factor In?
Research by Bryner and colleagues (1999) has demonstrated that one of the most important benefits of resistance exercise in a weight loss program is the preservation of muscle mass, even on very low calorie diets. In addition, Andrew Hill (2004) has shown that diet-only programs can lower a person's resting metabolic rate (RMR) by 20% (which may mean that approximately 300 fewer calories are expended per day). Bryner's research demonstrates that resistance training is one of the best protective interventions to maintain the RMR during a calorie-restricted weight loss program.

Fat-Burning Solution
Your clients may be gratified to know that with consistent endurance exercise, they truly will develop better "fat-burning furnaces" (mitochondria) in their bodies! Try to consistently focus workout designs on burning the most calories possible with the exercise plan—whether through harder workouts; easier, longer-duration workouts; or a combination of the two. In addition, remember that among the many benefits of resistance training is the fact that it helps preserve the body's muscle mass and RMR in calorie-restricted states. Consistently remind your clients, "To burn more fat, burn more calories."

Want more from Len Kravitz?


Bryner, R.W., et al. 1999. Effects of resistance vs. aerobic training combined with an 800 calorie liquid diet on lean body mass and resting metabolic rate. Journal of the American College of Nutrition, 18 (1), 115-21.

Hill, A.J. 2004. Does dieting make you fat? British Journal of Nutrition, 92 (Suppl. 1), S15-18.

Horowitz, J., & Klein, S. 2000. Lipid metabolism during endurance exercise. American Journal of Clinical Nutrition, 72 (2, Suppl.), 558S-63S.

Thompson, D.L., et al. 1998. Substrate use during and following moderate- and low-intensity exercise: Implications for weight control. European Journal of Applied Physiology and Occupational Physiology, 78 (1), 43-49.

Fitness Journal, Volume 4, Issue 8

Find the Perfect Job

More jobs, more applicants and more visits than any other fitness industry job board.

© 2007 by IDEA Health & Fitness Inc. All rights reserved. Reproduction without permission is strictly prohibited.

About the Author

Len Kravitz, PhD

Len Kravitz, PhD IDEA Author/Presenter

Len Kravitz, PhD, is the program coordinator of exercise science and a researcher at the University of New Mexico in Albuquerque, where he recently won the Outstanding Teacher of the Year award. Len was also honored as the 2006 Fitness Educator of the Year by the American Council on Exercise.